1. Field of the Invention
This invention relates to a quantity-of-light adjusting device for adjusting a quantity of light by driving an iris, or to an optical apparatus having the quantity-of-light adjusting device.
2. Description of the Related Art
The video apparatuses such as video cameras have made conspicuous advancement of late. As a result, these apparatuses, especially camera-integrated video tape recorders, have come to be equipped with automatic quantity-of-light adjusting devices in general.
The conventional quantity-of-light adjusting method which has been employed for such a camera-integrated video tape recorder is as described below with reference to FIG. 5:
Incident light representative of an image of a shooting object comes through a lens optical system 1. The incident light is imaged on the pickup image plane of an image sensor 3 after the quantity of the light is adjusted by means of iris blades 2 of a quantity-of-light adjusting mechanism (hereinafter referred to as iris). The object image which is thus formed on the image sensor 3 is photo-electrically converted into an pickup image signal. The pickup image signal is sent to a camera signal processing circuit 4. The camera signal processing circuit 4 performs various processes such as a gamma conversion process, etc., on the image signal to obtain a chrominance signal C and a luminance signal Y.gamma. as a video signal. The video signal is further processed by a camera encoder 5 which conforms, for example, to the NTSC system and is then outputted to the outside from a camera part in the form of a composite video signal.
Meanwhile, a luminance signal Y obtained also at the camera signal processing circuit 4 is sent to a detection circuit 6 for generating a control signal for the purpose of controlling the iris blades 2 in such a way as to make an exposure apposite to the luminance state of the image plane. The luminance signal Y is sent also to an AF (automatic focusing) block 7 for focusing. At the detection circuit 6, the luminance signal Y is subjected, for example, to an integrating detection process. After the integrating detection, the luminance signal is sent to a comparison circuit 8 which is disposed within a microcomputer 14. The comparison circuit 8 compares the luminance signal with a reference value (an apposite exposure value). A difference thus obtained is sent to a multiplier 9. The multiplier 9 then multiplies the difference by a coefficient corresponding to the position of the iris blades 2. The output signal of the multiplier 9 is sent from the microcomputer 14 to a driver 10. The driver 10 makes this signal from the microcomputer 14 into an apposite voltage. The driver sends the voltage to an actuator 11 to drive the iris blades 2.
The actuator 11 and the driver 10 are arranged as shown in FIG. 6. The iris driving signal outputted from the microcomputer 14 is integrated by an operational amplifier 21 and is then applied to a driving coil 22 of the actuator 11. As a result, a magnetic field is generated at the driving coil 22. The magnetic field causes a magnet rotor 23 to rotate. An arm 24 is mounted on the rotor 23. The tip of the arm 24 then moves the iris blades 2 to vary the aperture position of the iris. Further, the arm 24 is pulled by a spring 25 and its movement is arranged to be brought to a stop by one of stoppers 26. When the rotor 23 rotates, an electromotive voltage is generated at a damping coil 27. The voltage is fed back to the operational amplifier 21 for feedback control over the iris. Further, a magnetic flux density for a Hall element 28 changes when the rotor 23 rotates. The output of the Hall element 28 comes to the operational amplifier 29 to be amplified there. The amplified output of the Hall element 28 is sent to the microcomputer 14 as information on the position of the iris blades 2, i.e., as an aperture value.
In order to arrange a video camera in a smaller size, the size of a quantity-of-light adjusting device also must be reduced. However, the reduction in size of the quantity-of-light adjusting device makes it hardly possible to have a sufficient number of turns of the damping coil. Then, it becomes hardly possible to obtain an adequate performance for adjustment of the quantity of light.
To solve this problem, a device has been proposed as disclosed in Japanese Patent Application Laid-Open No. HEI 2-239782. This device is arranged to omit the damping coil and to perform feedback control over the speed of the rotor by using, in place of the damping coil, the output of the Hall element which detects the angle of rotation of the rotor. However, analog information obtained from the analog signal of the Hall element is directly used for the feedback control.